Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells. A target cell responds to a hormone because it bears receptors for the hormone.
In other words, a particular cell is a target cell for a hormone if it contains functional receptors for that hormone, and cells which do not have such a receptor cannot be influenced directly by that hormone. Reception of a radio broadcast provides a good analogy. Everyone within range of a transmitter for National Public Radio is exposed to that signal (even if they don't contribute!). However, in order to be a NPR target and thus influenced directly by their broadcasts, you have to have a receiver tuned to that frequency.
Hormone receptors are found either exposed on the surface of the cell or within the cell, depending on the type of hormone. In very basic terms, binding of hormone to receptor triggers a cascade of reactions within the cell that affects function. Additional details about receptor structure and function are provided in the section on hormone mechanism of action.
A traditional part of the definition of hormones described them as being secreted into blood and affecting cells at distant sites. However, many of the hormones known to act in that manner have been shown to also affect neighboring cells or even have effects on the same cells that secreted the hormone. Nonetheless, it is useful to be able to describe how the signal is distributed for a particular hormonal pathway, and three actions are defined:
Endocrine action: the hormone is distributed in blood and binds to distant target cells.
Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.
Autocrine action: the hormone acts on the same cell that produced it.
Two important terms are used to refer to molecules that bind to the hormone-binding sites of receptors:
Agonists are molecules that bind the receptor and induce all the post-receptor events that lead to a biologic effect. In other words, they act like the "normal" hormone, although perhaps more or less potently. Natural hormones are themselves agonists and, in many cases, more than one distinct hormone binds to the same receptor. For a given receptor, different agonists can have dramatically different potencies.
Antagonists are molecules that bind the receptor and block binding of the agonist, but fail to trigger intracellular signalling events. Antagonists are like certain types of bureaucrats - they don't themselves perform useful work, but block the activities of those that do have the capacity to contribute. Hormone antagonists are widely used as drugs.
Finally, a comment on the names given hormones and what some have called the tyranny of terminology. Hormones are inevitably named shortly after their discovery, when understanding is necessarily rudimentary. They are often named for the first physiologic effect observed or for their major site of synthesis. As knowledge and understanding of the hormone grow, the original name often appears inappropriate or too restrictive, but it has become entrenched in the literature and is rarely changed. In other situations, a single hormone will be referred to by more than one name. The problem is that the names given to hormones often end up being either confusing or misleading. The solution is to view names as identifiers rather than strict guidelines to source or function.
